A refining element is generally formed with a pattern of bars and intermediate grooves. The bars and grooves are formed in different ways, depending on which fibrous material is worked and the degree of working desired, and thus in the case of lignocellulosic material, which pulp quality is desired. The bars, for example, can be continuous or discontinuous, and can be arranged in different patterns. The working of the fibrous material is carried out substantially by the bars of the refining elements. The refining gap is formed so that the fibrous material, as seen in the radial direction, passes from the inside outwardly. Farthest inwardly in the refining gap, in an inner portion, or the in-feeding zone, the refining elements are normally formed with wide bars (coarse pattern) for bringing about a first disintegration of the material and for advancing the material outwardly in the refining gap. A certain defibering, i.e. a separation of the fibers of the material, also takes place in the inner portion of the refining gap, where the distance between the refining surfaces is the greatest. The refining gap decreases thereafter outwardly, i.e. the distance between the opposed refining surfaces decreases and transforms to a refining zone, at the same time as the bars become thinner (finer pattern), in order to achieve a desired working or refining of the fibrous material. The bars in the outer portion are placed at a greater density, which implies more bar edges for effecting the main working of the material. The outer portion can comprise more zones, in which case the pattern is usually made tighter from one zone to the other, radially outwardly.
In the transition from the inner portion with the coarse pattern to the outer portion with the finer pattern, the material is slowed down. This implies compression of the material and increased load on the bars of the refining element in this position. As a result thereof the bars of the refining element in the transition between the inner and outer portion of the refining element are subjected to increasing wear.